1. Technical Field
Dental prostheses and apparatus and methods of manufacturing them. In particular, computer-implemented methods of manufacturing dental prostheses, a computer-aided system for manufacturing dental prostheses, and dental prostheses made by the system and method.
2. Description of Related Art
Heretofore, the manufacturing of dental prostheses has been a highly labor intensive process requiring multiple fittings to a patient in need of them, and many steps that must be performed at the hands of skilled artisans. The dental prostheses may be a complete upper and/or lower set of prosthetic teeth and their mountings, i.e., dentures, or partial dentures, crowns, bridges, and the like.
By way of illustration, the following are the steps currently practiced in many “dental laboratories” for the fabrication of a conventional fixed dental prosthetic known as a crown:                1) A dentist prepares the tooth (or teeth) to be fitted with a fixed prosthetic by removing tooth structure that is decayed or to allow for space needed by the prosthetic device.        2) An accurate impression of the patient's existing gums and prepared teeth is made by the dentist at the dentist's office.        3) Gypsum material is poured into the impression to form a model (replica) of the dentition to be treated.        4) Wax is typically used to make a coping (thin metal substructure) on the model.        5) Using the “lost wax technique”, the wax is invested (covered) by a phosphate investment material and then it is heated to burn-out (remove) the wax, leaving a void in its place.        6) Metal is cast into the void created by the loss of wax to create a metal coping.        7) The metal coping is finished with grinding stones and typically heat-treated.        8) Porcelain powder dispersed in water is painted onto the metal coping.        9) The porcelain is fired in a furnace to sinter it into a continuous hard coating, resulting in the finished crown.        
It can be seen that in the above highly labor-intensive process, each of these steps introduces a potential for a processing error. Even the slightest error, such as the investment being too cool, or the powder/water ratio of the investment being incorrect may cause the crown to fit too tightly in the patient's mouth, resulting in improper occlusion (upper and lower teeth engagement). The crown may thus have to be scrapped or reworked through at least one iteration of additional process steps at considerable cost to the patient, dentist, and/or manufacturing lab.
Currently, Computer Aided Design and Computer Aided manufacturing (CAD/CAM) for “fixed” restorative dentistry has evolved to the point where a digital impression can now be made in the dentist's office and the entire process can be computer implemented. However, certain shortcomings still remain in fixed restorative dentistry as presently practiced. For example, subtle irregularities often found in anterior (front) teeth are difficult to replicate using CAD/CAM processes. Manual methods of making anterior fixed prosthetics enable unlimited aesthetic options, only limited by the creativity of the artisan (dental laboratory technician). Some CAD/CAM techniques involve the use of milling a monolithic block of ceramic that does not deliver optimal aesthetics (example: too opaque), especially for anterior applications. For example, most natural teeth exhibit translucency and subtle color variations. A common solution for this problem is for a dental technician to apply a stain and/or glaze of porcelain over the prosthetic made by CAD/CAM. However, this manual step may defeat the primary benefit of CAD/CAM: precise dimensional accuracy.
With regard to the manufacturing of removable dental prosthetics, such as dentures and partials, implementation of CAD/CAM has begun to occur. A key technology that is used in CAD/CAM denture manufacturing is “fused deposition modeling” (FDM). In FDM, a computer-controlled machine builds a three dimensional part by ejecting microscopic droplets of material while repeatedly traversing in an x-y plane, building the part layer-by layer. In a sense, the machine “ink-jet prints” each layer, and hence FDM is also referred to as “3D printing.” The physical model is built according to a three-dimensional virtual model that is prepared using CAD software and uploaded to the FDM machine.
CAD/CAM systems have recently been developed and used for the fabrication of partial denture frameworks. One such system uses a “haptic” device, which mimics a waxing tool that is familiar to dental technicians. However, this system generates only a CAD replica in plastic (made by a 3D printer), which requires subsequent extensive processing to obtain a metal partial denture framework. Hence there are still many error-prone steps after the CAD replica is made that can result in a poorly-fitting partial denture framework.
There have been some efforts by major manufacturers of dental materials to make a system to produce a complete (full) denture by 3D printing. The system includes a 3-dimensional scanner for scanning an impression, software for creating a 3-dimensional model of the denture, and the fused deposition modeling equipment for “printing” the denture. However, the materials available to use in 3-dimensional printers are neither as dense nor cross-linked like a normal plastic artificial tooth. Hence a problem remains with the resulting dentures because the denture teeth that are made with available 3D printing plastic materials are not sufficiently wear-resistant.
An alternative approach to denture fabrication is to first make a denture base using a milling machine, which may be computer controlled. Sockets are then milled by the machine into the denture base, and pre-fabricated artificial teeth are placed into the sockets. A problem with this approach is that most of the teeth must be adjusted to some extent to fit within the space required in order for the denture to properly occlude with the opposing arch of the opposing denture or the patients existing opposing teeth. Manual labor is required for the adjustment of teeth; therefore, the potential for errors is introduced into the manufacturing process.
Another problem with this method is that artificial teeth are not consistently sized. They are made from a molding process, with the molds being used for many years. Over the course of use, material from the wall of the mold will wear away, resulting in a mold cavity increasing in size. Hence a tooth made from a mold that has been in service for ten years will be larger than a tooth made when the mold was new. Additionally, molds contain multiple cavities, and the wear is not necessarily uniform. Thus the combination of wear with time and non-uniform wear results in the production of teeth that vary dimensionally within any given tooth size. Moreover, in the denture fabrication marketplace, artificial teeth are returnable for credit. It therefore becomes highly probable that artificial teeth produced 20 years ago from a new mold are in circulation with teeth produced very recently from the same but aged mold having different dimensions.
There is thus a problem in that the dimensional variation of artificial teeth is significant with respect to the dimensions of the sockets formed by the milling machine in which the teeth are to be fitted. The sockets must be milled sufficiently large so as to receive the largest tooth encountered within a given tooth size and shape (i.e. incisor, canine, molar, etc.), and countermeasures taken when the tooth is too small and does not fight tightly into its socket. One countermeasure is to use an acrylic repair resin to secure the teeth into position and to fill the gap(s), of various sizes that may be present around an undersized tooth.
However, this practice is undesirable. Additional labor is required for this step, which is costly and which is likely a manual process which can introduce potential errors to the denture fabrication. The risk of denture tooth “pop-outs” (debonding from the denture base) is more likely because the volume of bonding material is quite small relative to the conventional method of bonding denture teeth, and the bonding surface may be restricted to the circumference of the denture tooth which interfaces with the denture base (and limited bonding of the area of the tooth that opposes the occlusal surface because this area has been adjusted to rest on the “floor” of the socket). In the conventional approach, uncured denture base material surrounds the neck of the teeth and the area of the teeth that oppose the occlusal surface and chemical bonds are formed due to the volume of material and time that the uncured material is allowed to form cross-linked chemical bonds with the artificial teeth.
In addition, like the conventional approach, the patient will not see the final configuration of the denture until the delivery appointment, at which time the patient may reject the denture based on esthetics.
A further reason that “pop-outs” will be more likely with this approach vs. the conventional approach is that the conventional approach relies on a dental technician to adjust each artificial tooth in a way to optimize retention. For example, a dental technician will remove the “glaze” from a denture tooth (shiny and hard surface of the tooth created from a metal mold) to form a better bond with the denture base. Also, “diatoric” holes are often cut into the bottom or side of the tooth, or both, to allow acrylic material to flow in an optimal path to increase the surface area and create mechanical retention in a tooth. The step to provide diatoric holes is yet another processing step that increases cost and introduces the potential for further errors, such as artificial tooth fracture.
Yet another approach to denture fabrication is to mill blocks of polymerized plastic to make a complete denture. This process involves milling a block of pink methacrylate material as the denture base (including the gingiva surrounding the teeth). The teeth are then milled from a single piece of plastic. Lastly, the pink denture base and the milled teeth are cemented together. This technique is useful to make an immediate denture for temporary use, such as after a tooth-extraction for use while the gums heal. However, it is not suitable for long-term dentures because the artificial teeth made in this manner look unaesthetic. Natural dentition has subtle color (hue) variations as well as translucencies, color volume and defects. These effects are built-into most artificial teeth which are generally made in 2 to 4 layers of overlapping material (plastic or porcelain), each layer having different shades and levels of translucency. These layers create a natural effect of tooth structure, especially in anterior (front) teeth which often display “mamelons” and translucent incisal edges.
Artificial teeth that have an aesthetically pleasing appearance are generally made of highly cross-linked polymethylmethacrylate plastic, but may also be made of porcelain. Such artificial teeth are made with a series of metal dies in which the teeth are formed one-layer at a time. When all of the layers are completed, the “green” tooth is then heated to polymerize the plastic (or super-heated in the case of porcelain teeth). The heating process completes the cross-linking process in plastic teeth to make the teeth resistant to wear from the forces of mastication. This process is not compatible with the above overall denture fabrication process in which the full set of teeth are milled from a single piece of plastic and bonded to the milled denture base.
In summary, there remains a need for a method and apparatus for fabricating a denture at low cost in a minimal number of steps and with minimal manual labor, and preferably at a single manufacturing station. A denture made by any such method and apparatus must be made with sufficient precision so as to fit the patient properly, and have teeth that are firmly retained, wear resistant, and aesthetically pleasing.